Novel method of improving the mechanical properties of powder metallurgy parts by gas alloying

ABSTRACT

The following specification describes a process for improving the hardness and other mechanical properties of iron and steel Powder Metallurgy (P/M) parts. The first stage of the novel process consists of heating to and holding at a temperature between 590° C. to 720° C. unalloyed or low alloyed P/M parts in an atmosphere containing a Nitrogen donor such as Ammonia in either batch or continuous furnaces. The concentration of ammonia during the first stage is maintained between 3% to 15%. The second stage of the inventive process is an ‘aging’ process which may be conducted either as an in-line process or as a stand-alone independent process that involves the heating of P/M parts that have fully or partially cooled after the first stage to a temperature between 180° C. and 660° C. in an atmosphere of plain air or Nitrogen. The first stage may be performed in varying concentrations of the nitrogen donor wherein the temperature and time duration may also be varied to control the depth of hardening in the said part. The conditions may be optimized to achieve through hardness of the part without embrittllement. The optional stage two of the technology is an aging process that does not involve “quenching,” thereby significantly lowering distortion of treated parts and eliminating pollution associated with liquid quenching. The technology improves process economy by using low temperatures and consequently fuel consumption.

The following specification describes a process for improving thehardness and other mechanical properties of iron and steel PowderMetallurgy (P/M) parts in which the first stage is the alloying of theparts with Nitrogen which causes the formation of an austenitic phase inthe metal matrix of the parts in addition to the formation of hardtransformation products and interstitial Nitrogen throughout the sectionthickness of the parts or to a substantial depth below the surface ofthe parts depending on process parameters employed such as temperature,time at temperature, composition of the atmosphere gas mixture and theproperties of the P/M parts such as density, thickness and alloyingelements.

This specification also describes a subsequent second stage of “aging”the P/M parts which converts the bulk of the above mentioned austeniticphase to hard transformation products which are crystalline structuresincluding but not restricted to structures commonly referred to asbainite&martensite. This increases the hardness of P/M parts. Thestrength of P/M parts with a lower carbon content increases while thestrength of parts with higher carbon content reduces in which case thesecond stage of the process is performed only in applications where thestrength of the P/M part is not primary to its application. The secondstage is a novel aging process wherein the tradition quenching processhas been eliminated to reduce part distortion among other benefits suchas lowering of environmental pollution.

FIELD AND USE OF INVENTION

Several iron & steel components and parts in various areas ofapplication which include automotive, machinery, hardware, sports,firearms and domestic appliance are made by the powder metallurgy route(these being known as P/M parts) as the cost of production is lower thanother production processes. However P/M parts are inherently porous andthis makes them less suitable for use in stressed applications unlessthey are hardened and strengthened in a variety of ways which includealloying with expensive elements, increasing part hardness and densityby the process of heat treatment, impregnation and by mechanicalworking. Each of these processes has some disadvantages which includeadditional cost, environmental pollution and part distortion.

The said inventive process is a method of hardening and strengtheningP/M parts by gas alloying and this process does so in a manner thatreduces or eliminates several disadvantages of traditionally practicedprocesses. This novel process offers a radically different approach tohardening of P/M parts either as a conjunct process to the priorsintering process in which the P/M parts are actually manufactured or asan independent stand alone process.

This process has the benefits of lower energy consumption, lowerpollution, less shape distortion and reduction in the use of expensivealloying elements.

PRIOR ART

Iron and steel P/M parts are widely employed in automotive, machinery,hardware, sports, firearms and domestic appliances. Powder metallurgy isa manufacturing method where metal powders are compacted in a die cavitythat is nearly the shape of the final product prior to being “sintered”during which the powders form a bond, get consolidated into a singlemonolith and shrinks to the final shape with least machiningrequirement. The powder metallurgy manufacturing process offers manyadvantages which cannot be attained by other metalworking processes suchas highest raw material utilization, least machining, high qualityconsistency and lowest manufacturing costs for parts where P/M is afeasible manufacturing process.

Due to the nature of the manufacturing process, P/M parts do not attainfull density, have porosity and are consequently unsuitable forstressful applications. The strength & hardness of P/M parts can beimproved by heat treatment processes which are well known in the art.

Heat Treatment of P/M parts is no different to the heat treatment ofwrought parts, whether forged, stamped or cast and then machined and isdone by heating them in a controlled atmosphere to a temperatureslightly higher than the upper transformation temperature of the alloythe part is made of, usually above 800° C. followed by rapid cooling,normally by quenching in a liquid, usually oil, to obtain a hardmartensitic and/or bainitic structure in the case of parts with adequateCarbon content. The quenched parts are then tempered to reducebrittleness.

In the case of low alloyed and unalloyed low P/M low carbon parts theprocess of hardening as generally described above is performed but in anatmosphere that donates Carbon, such process being called Carburisingand in several cases with the addition of Nitrogen, such process beingcalled Carbonitriding, both of which are referred to as case hardeningprocesses as only the surfaces of the treated parts, to the depth ofdiffusion of Carbon/Nitrogen is hardened, the core being softer.

The surface treatmented P/M parts are quenched usually in oil and thereis a need to remove the oil from the surface and interior of the P/Mparts and this is an expensive and polluting process involving eitherwashing in a heated liquid detergent medium which generates pollutingeffluent or by burning which causes air pollution. Advanced processesthat reduce pollution such as vacuum de-oiling add significant cost tothe process.

Of more recent origin is the use of vacuum furnaces for hardening,carburizing and carbonitriding of P/M parts where quenching is done byrecirculation and cooled gas under pressure. While this processeliminates the pollution of oil quenching it nonetheless is an expensiveplant and an extra manufacturing step. Additional cost is incurred bythe necessity of having to use expensive alloying elements in the P/Mpart so the desired hardness can be obtained by gas quenching.

The heat extraction from the P/M part while quenching depends upon amongother factors, the part section thickness. Non uniform cooling of a partwith thick as well as thin sections results in dimensional distortiondue to non-uniform cooling.

Nitriding & Nitrocarburising are surface treatments which do notnormally involve quenching except in some cases. The difficulty ofapplying these processes to P/M parts is that a defined “case depth” isnot easily obtained due to the porous nature of P/M parts which causesnon uniform penetration the remedies to which are either an additionalprior process of porosity sealing or the relatively more expensiveprocess of plasma nitriding.

Sinter hardening is another hardening process where the parts arerapidly cooled as they emerge from a continuous atmosphere sinteringfurnace by the impingement of re-circulated & cooled furnace atmosphere.However as the heat extraction characteristics of gas is significantlylower than that of liquid quenchants, the P/M parts have to besufficiently alloyed with expensive elements such as Nickel & Molybdenumwhich increase the hardenability of the part but also make the part moreexpensive.

As is evident, all known hardening techniques in industrial practicesuffer from a variety of disadvantages and there is present a need foran improved technique which is the subject matter of the said inventiveprocess technology.

The technology described herein in detail, offers a solution that hasnever been attempted by other workers in the area of powder metallurgyfamiliar with the art. It is a completely different genre when it comesto treatment of P/M parts for improved strength and hardness. A thoroughsearch of patent literature has not found any document that matches thedisclosure either in concept, content or spirit. The closest referencewas found in a scientific paper by X, Yang & C. Kong & Y. Uiao entitled“A study on austenitic nitrocarburising without compound layer”presented at 2nd International Conference of Carburizing and Nitriding [Proceedings of the second International Conference on Carburizing andNitirding with Atmoshphere, 6-8 Dec. 1995, Cleveland, Ohio]. Howeverthis paper pertains only to the processing of wrought parts, not P/Mparts. Additionally the paper describes only a conventional shallowsurface hardening process & not a process where the part is hardenedthroughout its section thickness or to a substantial depth below thepart surface. It also does not describe the improvement in propertieswhich result from subsequent aging.

The technology described in its entirety in the subsequent sections willamply disclose the novel features and the intended utility of thisunique hardening method developed for use on P/M parts.

OBJECT OF THE INVENTION

The principal object of the invention is to devise a hardening andstrengthening process suitable for all types of iron & steel P/M partsincluding those with little or no hardenability enhancing alloyingelements.

Another objective is to achieve hardness and strength by the formationof nitrogen rich austenite in the metal matrix of the P/M parts eitherthroughout their section thickness or to a substantial depth below thepart surface.

Another object of the invention is to exclusively create said nitrogenrich austenite in the metal matrix of the P/M part where subsequentaging of the processed part is optional and may be eliminated whereaging is found to be unnecessary.

Another objective is to ‘age’ the P/M parts to convert the Nitrogen richaustenite to hard transformation products for improved hardness andstrength.

One more objective of “aging” is to harden parts with a lower carboncontent for internal strength and hardness and an increase in hardnesswithout an increase in strength of parts with a higher carbon content.

Yet another object of the invention is to make negligible the formationof embrittlling iron nitrogen compounds when performing the said novelprocess.

Yet another object of the invention is to create iron nitrogen compoundson the surface when performing the said novel process where requiredwithout causing embrittlment.

One more object of the invention is to improve the hardness andmechanical properties of P/M parts subjected to the said novel processwithout rapid cooling or quenching

Another object of the invention is to foster versatility to the saidnovel inventive process by enabling performance of the process incontinuous or batch furnaces or in furnaces that work at or aboveatmospheric pressure as well as vacuum furnaces.

Another object of the invention is to provide a versatile process thatmay either be performed in a module attached to the furnace or may beperformed in a furnace attached to or in line with another furnace.

Another object of the invention is to reduce the distortion of P/Mparts.

Another object of the invention is to lower the amount of energyinvolved in the hardening of P/M parts by the use of low processtemperatures and consequently reduces costs.

Another object of the invention is to lower the amount of environmentalpollution involved in the hardening of P/M parts.

STATEMENT OF THE INVENTION

Accordingly, the invention provides a novel process for increasing thehardness and strength of unalloyed and low alloyed P/M parts that ischaracterized by the use of low temperature gas alloying in an Ammoniacontaining atmosphere which causes the diffusion of Nitrogen into themetal matrix of the parts resulting in the formation of Nitrogen richaustenite.

The said novel process may be optimized by performing the process in oneor more steps where conditions of time between half and twelve hours,temperature between 590° C. to 720° C. and Ammonia concentration between3 to 15%, are varied individually or severally.

The vital differentiator for the said novel process is that it ispossible to either achieve diffusion and consequently strength andhardness throughout the section thickness of P/M parts or to a specifieddepth below the part surface by controlling the said process parameterswithin said range.

The parts initially hardened by the said Nitrogen gas alloying processin the first stage may be further subjected to an optional second stageof aging for additional hardening and strengthening of P/M parts withlower carbon content or hardening without strengthening of parts with ahigher carbon content by heating the P/M parts to a temperatures between180° C.-590° C. and holding them at temperature for a time periodgenerally not exceeding two hours.

One advantage of the said novel process is that it replaces theconventional method of strengthening and hardening P/M parts by heatingthe parts to above 800° C. followed by rapid cooling or quenching,frequently in oil.

This novel adaptable process technology reduces part distortion (as iteliminates rapid cooling from a high temperature), fosters betterprocess economics by reducing energy utilization (as lower processestemperatures are employed) and eliminates pollution associated with thetraditional oil quenching process.

Another advantage is that either one or both stages of the novel processmay be performed in any type of furnace and may be performed as anindependent stand-alone process or as an in-line process.

DESCRIPTION OF THE INVENTION

The first stage of the process according to the invention consists ofone or more steps of different combinations of temperature, gascomposition and time performed to achieve diffusion of nitrogen into themetal matrix of P/M parts so as to primarily cause the formation ofnitrogen rich austenite along with a certain amount of hardtransformation products and interstitial Nitrogen, which increases thehardness and strength of the parts. An additional and subsequent secondstage of aging is optionally performed on parts processed as disclosedabove in the first stage, to convert this nitrogen rich austenite intohard transformation products and thereby further increase the hardnessand strength of low carbon containing P/M parts and increase thehardness without a corresponding increase in strength of parts with ahigher carbon content. The first stage of gas alloying can be performedin-line with the prior sintering process, in a module attached to thesintering furnace or as an independent stand alone process in a separatefurnace. The second stage of aging can be similarly performed in-linewith the first stage of the process or as an independent stand aloneprocess. The process has been primarily devised to impart hardness andmechanical strength to all P/M parts including those with little or noalloying elements and without rapid cooling or quenching.

According to one embodiment of the invention the first and the secondstages of the said inventive process can be done as a conjunct process,in line with the prior process, sintering in case of the first stage andthe first stage of the process in the case of the second stage, eitherin the same furnace or in a module attached to the prior furnace or inanother furnace placed in line to the prior furnace.

According to another embodiment of the invention the first and thesecond stages of the said inventive process can be done sequentially butin different furnaces.

According to another embodiment of the invention either the first or thesecond stages can be done either in batch furnaces or in continuousfurnaces

In one more embodiment of the invention the process parameters in thefirst stage of the process can be controlled in one or more steps ofvarying time, temperature and atmosphere gas composition to optimize theprocess with respect to the characteristics of the P/M part, applicationof the part, logistics and process economics.

In one more embodiment of the invention the process parameters in thefirst stage of the process such as time, temperature and atmosphere gascomposition can be controlled to bring about nitrogen diffusionthroughout the cross section of the P/M part to the extent allowed bythe density and section thickness of the P/M part.

The first stage of the novel process consists of heating to and holdingat a temperature between 590° C. to 720° C. unalloyed or low alloyed P/Mparts in an atmosphere containing a Nitrogen donor such as Ammonia ineither batch or continuous furnaces. The concentration of ammonia duringthe first stage is maintained between 3% to 15%.

The second stage of the inventive process is an ‘aging’ process whichmay be conducted either as an in-line process or as a stand-aloneindependent process that involves the heating of P/M parts that havefully or partially cooled after the first stage to a temperature between180° C. and 660° C. in an atmosphere of plain air or Nitrogen or in theevent the second stage is combined with yet another process such as forexample, steam treatment, then in the atmosphere that such process iscarried out in, to effect conversion of nitrogen rich austenite formedduring the first stage of the inventive process to hard transformationproducts that further improves the strength and/or the hardness of theP/M parts depending on the carbon content of the parts.

This said first stage can consist of one or more steps where conditionsof time, temperature and gas composition, are varied individually orseverally to meet the demands raised by the application the P/M part isused for, the extent of alloying elements in the part, the density andsize of the part, the type of furnace employed, availability ofutilities as well as economic considerations and such variations do notaffect the scope of the claims as appended as they only allow dynamicuse of basic principles devised for achieving utility end points asstated in the objectives.

In one embodiment of the inventive process the Ammonia concentration ofthe process atmosphere can be varied (between 3%-15%) in different stepsof the first stage of the process while the temperature is keptconstant.

In another embodiment of the inventive process the Ammonia concentrationin the process atmosphere can be pulsed or changed at periodic intervalsin different steps of the first stage of the process while thetemperature is kept constant or also varied.

In another embodiment of the inventive process the Ammonia concentrationof the first stage of the process atmosphere can be kept constant whilethe temperature is varied in different steps of the process.

In another embodiment of the inventive process the atmosphere gascomposition the P/M part is exposed to either while being heated to orcooled from the process temperature to eliminate the presence of air canbe Nitrogen or any other inert gas in case when the process is carriedout without a plasma field. It is clarified that molecular Nitrogen willnot react with metal, for which nascent Nitrogen which comes fromcracking of Ammonia on the part surface is required.

In another embodiment of the inventive process the P/M part can beprocessed in vacuum while being heated to the process temperature andcan cooled down in vacuum or in Nitrogen or any other inert gas.

In one embodiment of the inventive process the temperature can be variedfrom 0.5 hour after the part has reached process temperature to 12 hoursdepending on the process temperature employed, the part size, the partdensity and the depth of nitrogen diffusion below the part surface thatis required.

The size, density, shape retention and chemical composition of P/Mparts, the process temperature, the process economics, the type andcapacity of the processing furnaces, the availability of utilities andproperties required in the P/M part are factors that govern the choiceof parameters to be applied when performing the said inventive process.Application of specific conditions are to be decided depending on one ormore of these factors and these have been clarified by way of theexamples provided below which are intended only to explain the novelprocess technology further. However persons skilled in the art wouldknow that such references would in no way limit the scope of theinvention as appended in the claims.

For example P/M parts of Iron with 2% Copper and 0.5% Carbon having adensity of 6.8 grams per cubic centimeter were subjected to a two stepprocess at a constant temperature of 675° C. in an atmosphere with anammonia concentration of 10% for one hour during the first step followedby a second step of another hour where the atmosphere gas is entirelyNitrogen without any Ammonia, to achieve an improvement in hardness from200 Vickers Hardness Scale in the sintered P/M parts before beingsubjected to the inventive process to above 320 Vickers Hardness Scalethroughout the cross section of the part without the formation of ironnitrides on the surface.

For another example P/M parts of Iron with 2% Copper and 0.5% Carbonhaving a density of 6.8 grams per cubic centimeter were subjected to asingle step process at a constant temperature of 700° C. in anatmosphere with an ammonia concentration of 5% for half hour to achievean improvement in hardness from 200 Vickers Hardness Scale in thesintered P/M parts before being subjected to the inventive process to650 Vickers Hardness Scale on the surface gradually reducing to theoriginal core hardness of 200 Vickers in a distance of 0.65 mm below theparts surface with the formation of metal nitrides on the surface.

For another example P/M parts of Iron with 2% Copper and 0.5% Carbonhaving a density of 6.8 grams per cubic centimeter were subjected to atwo step process at a constant temperature of 660° C. in an atmospherewith an ammonia concentration of 10% for 2.5 hours during the first stepfollowed by a second step of another 1.5 hours in an atmosphere with anammonia concentration of 3% to achieve an improvement in hardness from200 Vickers Hardness Scale in the sintered P/M parts before beingsubjected to the inventive process to above 492 Vickers Hardness Scaleon the surface gradually reducing through the cross section of the parttill 236 Vickers at the core, without the formation of iron nitrides onthe surface.

The second stage of the inventive process, the ‘aging’ process wasperformed at a temperature of 350° C. for a period of 2 hours in air forall the examples described above.

For yet another example three types of P/M iron parts all with 2%Copper, in the first case with 0.5% carbon and a density of 7.2 gramsper cubic centimeter, in the second case with 0.8% Carbon and a densityof 6.8 grams per cubic centimeter and in the third case with 0.9% Carbonand a density of 7.2 grams per cubic centimeter were subjected to a twostep process at a temperature of 650° C. in the first step of half hourwith an Ammonia concentration of 6% followed by a second step where thetemperature was maintained at 700° C. for half hour with an Ammoniaconcentration of 4%. All the processed parts were aged at temperaturesof 200° C., 350° C., 450° C. and 540° C. in air for a period of 1 hourin all cases. All parts were subjected to a ‘crush test’, a measure ofradial crushing strength and it was seen that parts with lower carboncontent (of 0.5%) the strength was significantly higher than thestrength of the ‘as sintered’ part after the first stage of the process.The strength first reduced as the aging temperature increased and thenincreased, in all cases being higher than the ‘as sintered’ part exceptin one case where it was equal to the strength of the ‘as sintered’part. Parts with a higher carbon content of 0.8% exhibited higherstrength after stage one of the process compared to the ‘as sintered’strength but lower at all aging temperatures. Parts with a higher carboncontent of 0.9% exhibited higher strength in the ‘as sintered’ conditionafter stage one of the process compared to the any of the parts thatwere processed. In all cases the hardness of all parts however processedwere higher than the hardness of the ‘as sintered’ parts.

The present invention is of course, is in no way restricted to thespecific disclosure found herein but will also include any modificationswithin spirit and the scope as appended in the claims.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 represent time temperature graphs of different embodimentsof the inventive processes.

FIG. 5 shows the microstructure of the surface of a P/M part which hasbeen subjected one embodiment of the novel process wherein hardtransformation products have been formed throughout its cross sectionwithout the formation of iron nitrides

FIG. 6 shows the microstructure of the core of the same P/M partmentioned above wherein hard transformation products are seen.

FIG. 7 shows the microstructure of another P/M part which has beensubjected to yet another embodiment of the said novel process wherein ashallow iron nitride layer is visible along with a substrate consistingpredominantly of hard transformation products.

FIG. 8 shows the core of the same P/M part described in FIG. 7, thatconsists predominantly of ferrite and pearlite with less hardtransformation products compared to the microstructure shown in FIG. 6.

FIG. 9 shows the hardness profile from surface to core of P/M parts thathave been subjected to some embodiments of the novel process that showthe increase in hardness throughout the cross section of the partscompared to the hardness of parts that have not been subjected to theprocess, shown as the flat line at the bottom and the hardness of partsthat have been subjected to conventional heat treatment process ofheating and quenching in oil, the two flat lines shown at the top of thegraph.

FIG. 10 shows the hardness profile from surface to core of P/M partsthat have been subjected to some other embodiments of the novel processthat show increase in hardness at the surface of the parts graduallyreducing towards the core of the parts compared to the hardness of partsthat have not been subjected to the process, shown as the flat line atthe bottom and the hardness of parts that have been subjected toconventional heat treatment process of heating and quenching in oil, thetwo flat lines shown at the top of the graph.

FIG. 11 Radial crushing strength of as sintered parts compared withsatgel & stage2 conditions

FIG. 12 is a photomicrograph of Gas alloyed parts (surface structure)after Stagel

FIG. 13 is a photomicrograph of the same FIG. 12 (core structure) afterStagel

FIG. 14 is a photomicrograph of the same FIG. 12 (surface structure)after Stage2

FIG. 15 is a photomicrograph of the same FIG. 12 (core structure) afterStage2

FIG. 16 is a photograph of a scanning electron microscope shownalongside an energy dispersive X-ray of the photograph which shows theNitrogen concentration on the surface of a P/M part subjected to oneembodiment of the novel process.

FIG. 17 is another photograph of a scanning electron microscope shownalongside an energy dispersive X-ray of the photograph which shows theNitrogen concentration on the surface of the P/M part subjected toanother embodiment of the novel process.

FIG. 18 is a graph of the Nitrogen concentration from the surface tocore of the P/M parts described in FIGS. 16 & 17.

BEST METHOD OF WORKING THE INVENTION

The said novel process technology has been devised for improving thehardness and other mechanical properties of iron and steel PowderMetallurgy (P/M) parts in which the first stage is the alloying theparts with Nitrogen gas which causes the formation of an austeniticphase in the metal matrix of the parts throughout the section thicknessor to a controlled depth beneath the surface of the parts, in additionto the formation of hard transformation products and interstitialNitrogen. This is followed in some cases by a second stage of “aging”which causes an additional improvement in hardness in all P/M parts thusprocessed as well as strength in P/M parts with less carbon content, bythe conversion of the bulk of the above mentioned austenite phase tohard transformation products. The first stage of the process can beperformed in one or more steps where time, temperature and atmospherecomposition is varied depending on the size, density and chemicalcomposition of the P/M part and the use the parts are put to. Thevarious permutations and combinations may be easily understood byreading the varying embodiments of the invention described above.Examples have been suggested to illustrate the various embodimentsdescribed that may be practiced to achieve the desired utility andadvantages provided by the inventive process.

I claim:
 1. A novel two stage process for heat treating unalloyed or lowalloyed iron & steel P/M parts consisting of two major stages whereinthe first stage is the alloying of the parts with Nitrogen gas whichcauses the formation of an austenitic phase in the metal matrix of theparts throughout the section thickness or to a controlled depth beneaththe surface of the parts, in addition to the formation of hardtransformation products and interstitial Nitrogen. This is followed inmost but not all cases by a second stage of “aging” the P/M parts whichcauses an additional improvement in hardness as well as other mechanicalproperties in P/M parts made in certain alloys.
 2. A novel inventiveprocess technology as claimed in claim 1 where the first stage may ormay not be followed by the second stage.
 3. A novel process as claimedin claim 1 & 2 wherein the first stage involves heating of unalloyed orlow alloyed P/M parts in an atmosphere containing a Nitrogen donor suchas Ammonia to a temperature between 590° C. to 720° C.
 4. A novelprocess as claimed in claim 1,2 & 3 wherein the first stage involvesheating of unalloyed or low alloyed P/M parts in an atmospherecontaining a Nitrogen donor such as Ammonia to a temperature between590° C. to 720° C. For a time duration between half an hour to twelvehours.
 5. A novel process as claimed in claim 1, 2, 3 & 4 wherein theAmmonia concentration is controlled between 3% to 15%.
 6. A novelinventive process as claimed in claims 1 to 5 wherein the stage oneprocess is conducted in a single step where the temperature and residualAmmonia concentration is kept constant for the duration of the process.7. A novel inventive process as claimed in claims 1 to 6 wherein thestage one process is conducted in multiple steps where the temperatureis maintained at different levels in each step.
 8. A novel inventiveprocess technology as claimed in claims 1 to 7 wherein the stage oneprocess is conducted in multiple steps where the concentration ofresidual Ammonia is maintained at different levels in each step.
 9. Anovel inventive process as claimed in claims 1 to 8 wherein thecontrolled atmosphere required in stage one of the novel processcontains in addition to Ammonia, Nitrogen or any other inert gas with orwithout Hydrogen.
 10. A novel inventive process as claimed in claims 1to 9 wherein the P/M parts are heated to the stage one processtemperature in an atmosphere of Nitrogen or any other inert gas with orwithout Hydrogen and with or without ammonia.
 11. A novel inventiveprocess as claimed in claims 1 to 10 wherein the P/M parts are cooledfrom the stage one process temperature an atmosphere of Nitrogen or anyother inert gas with or without Hydrogen inorder to avoid air
 12. Anovel inventive process as claimed in claims 1 to 11 wherein the P/Mparts are heated to and cooled from the stage one process temperature inan atmosphere containing Ammonia with Nitrogen or any other inert gasand with or without Hydrogen
 13. A novel inventive process for treatmentof low alloyed or unalloyed P/M parts as claimed in claims 1 thorough 12where the stage one process results in the alloying of the P/M partswith Nitrogen.
 14. A novel process for treatment of low alloyed orunalloyed P/M parts as claimed in claims 1 through 13 wherein Nitrogendiffusion results individually and severally in the formation of anitrogen enriched ferrite/pearlite phase, a nitrogen enriched austenitephase, hard transformation products and an iron nitride phase dependingon the gas composition used in stage one of the process.
 15. A novelprocess for treatment of low alloyed or unalloyed P/M parts as claimedin claims 1 through 14 wherein the stage one process temperature, theatmosphere gas composition and the process duration can be controlledindividually or severally on the basis of the size, density and chemicalcomposition of the P/M parts to control the required depth of diffusionof Nitrogen into the parts.
 16. A novel process for treatment of lowalloyed or unalloyed P/M parts as claimed in claims 1 through 15 whereinthe stage one process temperature, the atmosphere gas composition andthe process duration can be controlled individually or severally on thebasis of the size, density and chemical composition of the P/M parts toachieve diffusion of Nitrogen throughout the cross section of the partswith higher hardness at the surface and a hardness gradient that reducestowards the core of the part.
 17. A novel process technology fortreatment of low alloyed or unalloyed P/M parts as claimed in claims 1through 16 wherein the stage one process temperature, the atmosphere gascomposition and the process duration can be controlled individually orseverally on the basis of the size, density and chemical composition ofthe P/M parts to achieve diffusion of Nitrogen throughout the crosssection of the parts with higher hardness at the surface and a hardnessgradient that reduces towards the core of the part without the formationof embrittling iron nitrides.
 18. A novel process technology fortreatment of low alloyed or unalloyed P/M parts as claimed claims 1through 17 where the resulting increase in strength and hardness of P/Mparts subjected to stage one of the novel process is a result offormation of predominantly Nitrogen rich austenite with interstitialNitrogen and hard transformation products.
 19. A novel processtechnology for treatment of low alloyed or unalloyed P/M parts asclaimed in claim 1, where the stage one of the process as claimed inclaims 1 to 18 may be followed by stage two of the process in which P/Mparts may be aged so as to convert the predominant phase of Nitrogenrich austenite into hard transformation products.
 20. A novel processtechnology for treatment of low alloyed or unalloyed P/M parts asclaimed in claim 1, where the stage one of the process as claimed inclaims 1 to 19 is followed by stage two of the process in which P/Mparts are aged by heating them to a temperature between 180° C. to 660°C. in air or in Nitrogen or any other inert gas and holding the parts atthe aging temperature for a period not exceeding two hours.
 21. A novelprocess technology for treatment of low alloyed or unalloyed P/M partsas claimed in claim 1 wherein the first stage and the second stage maybe conducted as an in-line process together with the prior process,sintering of the P/M part in case of stage one and stage one in the caseof stage two where sintering, stage one and stage two can be carried outin continuous furnaces with material transport systems such as conveyorbelt, pusher tray, walking beam, roller hearth and rotary hearth as wellas batch vacuum furnaces or other batch furnaces which work atatmospheric or elevated pressure.
 22. A novel process technology asclaimed in claim 1 where hardening and strengthening of P/M parts isachieved without the necessity of heating and rapid cooling orquenching.
 23. A novel process technology as claimed in claims 1 to 22where environmental pollution caused by rapid cooling by quenching inoil and subsequent washing off the oil is averted.
 24. A novel processtechnology as claimed in claims 1 to 23 wherein the P/M parts sotreated, without quenching from a high temperature suffer lessdistortion.
 25. A novel process technology as claimed in claims 1 to 24where the cost of hardening P/M parts is reduced by the elimination ofrapid cooling or quenching.
 26. A novel process technology as claimed inclaims 1 to 25 wherein the energy consumption is lowered by lowering theprocess temperature.
 27. A novel process technology as claimed in claims1 to 26 where the cost of manufacturing P/M parts is reduced by theelimination of alloying elements required to increase the hardness andstrength of P/M parts.